This invention relates to a fluid circulation system for heat exchangers, and more particularly to a redundant heat removal system for heat exchangers used in nuclear reactor systems.
The nuclear reactor produces heat by fissioning of nuclear materials which are fabricated into fuel elements and assembled within a nuclear core. In commercial nuclear reactors, the heat produced thereby is used to generate electricity.
In a liquid cooled nuclear reactor, such as a liquid metal cooled breeder reactor, reactor coolant, such as liquid sodium, is circulated through the primary coolant flow system. A typical primary system coolant flow loop comprises a nuclear core within a reactor vessel, a heat exchanger, a circulating pump, and piping interconnecting the aforementioned apparatus. In nuclear reactors having more than one primary system coolant flow loop, the nuclear core and the reactor vessel are common to each of the primary system flow loops.
The heat generated by the nuclear core is removed by the reactor coolant which flows into the reactor vessel and through the nuclear core. The heated reactor coolant then exits from the reactor vessel and flows to the heat exchanger, where it transfers its heat to a flow system associated therewith. If the nuclear reactor system is of the direct type, this flow system coolant is water, which is transformed into steam as it passes through the heat exchanger, and is connected to turbines and electrical generators.
If the nuclear reactor system includes an intermediate coolant flow loop, the intermediate coolant, which may be a liquid metal or a liquid organic fluid, is heated, and flows to a second heat exchanger where its heat is transferred to a fluid of a secondary coolant flow loop. The secondary coolant would be water transformed into steam, and coupled to steam turbines and electrical generators.
In the nuclear reactor field, safety considerations mandate that redundant means be provided for removal of heat produced in the core in the unlikely event of a secondary coolant system failure, or for standby operations during which the secondary coolant system is not operating. In the prior art, these redundant means have taken two forms.
The first method was to provide an additional coolant flow loop to the nuclear core. In the event one of the coolant flow loops failed, this additional loop would become operable and would circulate hot reactor coolant through its system. Although this system provided a redundant means, it was economically unfeasible. The physical size of this additional heat removal system was comparable to the main heat transport loops, but it did not contribute to the plant power generation capability.
The second method used in the prior art was to make use of the main heat transport system for heat removal to a redundant water system through the steam generators. As such, it requires that the normal heat transfer systems be operable in an emergency condition. It also requires that redundancy be provided in the secondary coolant flow system, that a third loop be installed in the heat exchangers, and that a class I supply of water be stored within the plant. Although this method is also quite expensive, it is probably less expensive than that incurred with the first method. A second problem with this method, however, is that the operation of the plant for emergency or decay heat, removal operation is quite complicated, requiring large condenser flows, blow-off systems, and recirculating water systems.